The heat treatment (e.g. brazing, annealing, sintering etc.) of metals and alloys is most typically carried out at temperatures above 1000.degree. F. in an atmosphere containing both an inert and a reducing gas. Typically nitrogen (N.sub.2) is the inert gas and hydrogen (H.sub.2) is the reducing gas. The metal is first preheated in an inert atmosphere such as nitrogen with or without a small amount of hydrogen. The metal is then sent to a reaction zone where it is heat treated to the desired final temperature. The heat treated metal is then sent to a cooling zone, again containing an inert atmosphere such as nitrogen with or without small amounts of hydrogen. Nitrogen's only function is to keep air out of the interior or tunnel of the furnace through which the metal to be heat treated travels. Hydrogen is both a reducing gas and, like nitrogen, keeps air out of the furnace tunnel. The total amount of nitrogen and hydrogen required to keep air out of the furnace is primarily determined by the geometry and dimensions of the furnace tunnel and the size of the opening at each end.
All such atmospheres contain small amounts of impurities such as oxygen and moisture. These impurities react with the metals at above about 1000.degree. F. to produce unwanted surface metal oxides. This problem is usually corrected by increasing the proportion of hydrogen in the atmosphere while still maintaining the same total nitrogen and hydrogen atmosphere flow i.e. by increasing hydrogen and decreasing nitrogen.
Such heat treating processes suffer from one key disadvantage. Although nitrogen is clearly the most preferred inert gas, nitrogen tends to induce nitriding of the metal. It is extremely harmful to nitrogen-sensitive materials such as stainless steels, titanium or titanium containing alloys, refractory metals and materials containing refractory metals such as tungsten, vanadium, etc. Nitriding is the process by which layers of metal nitrides are formed on the surface of the metal as it reacts with nitrogen in the atmosphere at a temperature above some minimum temperature specific to the metal being heat treated. For most common metals, the minimum temperature for nitriding is about 1000.degree. F. Nitriding decreases corrosion resistance and toughness of the metal and produces an aesthetically unappealing dull/matte finish.
A method of addressing the nitriding problem is to heat treat metals with 100% hydrogen throughout the inside length of the furnace i.e. in the absence of nitrogen. However, this method suffers from a number of disadvantages. First, hydrogen is 3 to 5 times more expensive than nitrogen. Second, more heat is generated as escaping hydrogen is burnt at both ends of the furnace. This excess heat causes discomfort to operators working near the ends of the furnace. Third, the metal parts get reheated and oxidized as they exit the furnace under the flame where escaping hydrogen is burnt. Fourth, excess hydrogen usage in the furnace makes the operation inherently less safe. Fifth, 100% hydrogen may be detrimental to certain furnace components such as globars, belts, curtain materials, etc. Sixth, 100% hydrogen may adversely affect the heat treating operation itself; for example, during the brazing of metals, excess hydrogen may cause unwanted flow of filler metals (e.g. flashing).
Some of these disadvantages can be reduced by zoning the furnace atmosphere i.e. introducing 10 to 15% by volume nitrogen near each end of the furnace where the temperature is below about 1000.degree. F. At such low temperatures, there is little if any nitriding. The balance of the total atmosphere is still hydrogen and is introduced in the hot or reaction zone of the furnace. This substantially reduces the nitriding problem. However, 70 to 80% of the total atmosphere is still hydrogen. This is still too high an amount of hydrogen and therefore suffers from the disadvantages listed previously. It is desirable to reduce the overall hydrogen percentage to well below 50% by volume and in some cases well below 30%. If the amount of nitrogen introduced into the ends of the furnace is increased to 25% or more by volume to reduce the hydrogen content, then nitriding will take place to an undesirable degree.
Another possible solution is to keep the amount of nitrogen on either end of the furnace below about 15% of the total atmosphere and dilute hydrogen in the reaction zone with argon. Depending upon the amount of argon used, overall hydrogen can be brought down to desirable levels e.g. below 30% without losing the ability of hydrogen to reduce surface oxides. However, this method suffers from one key disadvantage i.e. the overall cost is too high as argon is 2-4 times more expensive than hydrogen which in turn is 3-5 times more expensive than nitrogen. Accordingly, industry has not favored the use of argon as described in the method above.
It would be a significant advance in the art of heat treating metals if a process could be devised in which the nitrogen level is lowered sufficiently in the reaction zone to avoid nitriding yet the overall cost of the process remains within commercially acceptable limits. It would be an even more significant advance if the process could be devised to both lower nitriding and still keep the overall hydrogen levels well below 50% by volume of the total atmosphere.